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One sick, one dead, brand new beautiful aquarium. What's going on?

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So, I just started my aquarium. Water temperature is about 79 degrees (goes down to 76 in the night though). I added four von rio tetras (about an inch long apiece) and let the tank sit for three weeks with them. Added three plants (a crispus, anubia, and a chilensis), and waited another two weeks. So it was five weeks from the start. I had the water tested weekly until this five week mark, when I saw there was no ammonia, nitrate, or nitrite present in the water. The cycle was done (said the person in the aquarium store), and I could put more fish in. So here's what I added. For those of you who read my intro, I just got out my list of their names):

1 Yo-yo Loach
1 Dwarf Frog
1 Bushynose Pleco
2 Panda Corys

So that was last Saturday. They all seemed fine. I feed once a day in the afternoon. My tetras always eat (I give them one flake each), the loach is digging around on the bottom and must be eating *something*, and the pleco is eating algae (I think). I haven't been able to observe the corys or the frog eating, but that doesn't mean they aren't as soon as I turn my back since the food disappears. I also drop in some frog food for the frog in addition to the flakes.

Yesterday I noticed a white blotch on the side fin of one of my corys. I didn't know what it was, so I left it alone for a day to see if it worsened. I just got home from class and that cory is dead. Now the other one has a whitened (but not a distinct white colour) patch on the scales where the adipose fin attaches to his body. I'm concerned.... I don't see myself as having done something wrong (please let me know if I have), and I wasn't aware that an illness could kill so fast.

One thing I do notice is one of my corys would swim around a lot, swimming against the sides of the tank, and crazy. The other is much calmer. I'm not sure which one died, because I can't tell them apart easily....

Can anyone shed light on the subject? If pictures would help I could give it my best shot (which isn't much.... no good camera). Really sorry for such a long post; I know it's a terrible way to make a first impression here. I'm just clueless. As far as I've been told, there's no "Read this book to become a total expert on fish care" book.

One of the problems with cycling with fish (as you have done) is that the filter only develops enough beneficial bacteria to support the fish you already have - that means that you can only add a few fish at a time to the tank so the bioload doesn't increase too rapidly... Your tank may have been cycled with 4 tetras in it, but by adding 4 more fish and a frog at one time you more than doubled the bioload.
You never mention in your post what size the tank is and what filtration you have on it, which would also have an impact on how many fish you can add safely at one time... for example, if you have a larger (say, 55 gallon) tank, adding 4 small fish would raise the overall ammonia concentration so slowly that your biological filter should have time to adjust, while adding 4 small fish in a small tank can be catastrophic.
I would guess you had an ammonia or nitrite spike that killed your fish (are you still testing for ammonia/nitrites/nitrates?) - or, if not outright killed, must have weakened them to make them more succeptable to disease.

You also have to remember that most of the fish at the store are often not in the best shape when you get them - most would have spent a couple of days being shipped in little baggies, and are now in cramped conditions in the store tanks. It is pretty common for them to come complete with diseases, and you should really consider getting a small quarantine tank so you can keep any new fish separate for a few weeks to make sure they are not carrying something that could pass onto your existing fish...

Finally, what are your plans for final stocking? this will, of course, depend on the size of the tank, but I hope you realise that most of the fish you have listed are social/schooling fish and need to be in groups of about 6 minimum (i.e. you would want at least 6 of the tetras, at least 6 of the panda cories, at least 6 of the yoyo loaches). Since both loaches and cories are bottom dwellers/feeders, it is possible that the stress of not being in a school/group could have caused some aggression in your tank between the loach and the cories, which could also have contributed to them getting sick.

At this point, I would recomend doing a signiffican water change (50%), and monitoring your remaining fish very closely. Feed them very lightly over the next few days, and don't add any more fish until you are sure that all the remaining ones you have are healthy, and that your tank is once again cycled.

Second problem: you added a pleco (high poop producer) and corys at the same time with a fish in cycle tank after just a few weeks. They should always be the last added because this is what happens... they (any catfish) need a matured tank with stablility (long term stability, not 2-3 weeks).

Whoa - I left out the critical details.... It's a 20 gallon tank, with an Aqueon 20 filter. Yes, I have been testing weekly. For the first five weeks I brought a water sample to the store (tomorrow makes six weeks), and I intend to continue to do that for quite some time (and buy a home test kit eventually). Last weekend there was a drop to very little ammonia, nitrate, and nitrite, and I was told by the guys there I could add about the number of fish I ended up buying. I don't know why they told me that, then. I have heard about a quarantine tank.... how many gallons would you consider small?

So, I'm totally new to this. I have no idea what I want to do, so I have really just been doing it as I go. I figure if I want to try something different I'll get a second tank when I know more. Until then, I picked the tetras for their colour. I got the pleco to eat algae, and the loach (which I have become fond of) to eat snails. I'm just trying to figure out what I'm doing, and have an aquarium with some nice fish. I definitely want to continue using live plants, but that's about my only criteria for this tank (other than keeping my fish healthy).

You say a school of six - should I be getting another few tetras when the tank is cycled again? As for the loach, I read online that loaches grow to 4.5in long. I can't imagine six of them in a 20 gallon tank.

It's interesting you say that. They didn't mention that in the store.... actually specifically told me the opposite. However, I did notice the cories chased the loach around a little. My second cory's fins are getting dark in colour now as well. I just noticed that.

I'm changing the water now, no problems on that front (and dechlorinating before adding it to the aquarium, not after as I've been told that's bad for the fish).

Edit:
@Sandz They don't have white specks, it's large white patches.... from pictures I've seen, it doesn't look like Ick. Could be wrong.

That would have been great to know about the catfish before I added them to the tank. It's been 6 weeks, but you're right, that's not very long. Where can I learn more about what each fish species needs so I can make an informed descision when adding fish to the aquarium?

Oh wow, a 20gal tank? You really do have a problem. The tank its self will not produce algae at the level to support a pleco. Also wood is a large part of their diet. You have a 5 week old tank, I cannot see how you have a snail problem that requires a fish specifically for it. And you are 100% right on there being 6 of them in a 20... they dont match.

I would stock 6-10 of one species of tetra, 6-10 corries and 1 centerpiece fish for this tank and that be it. What you have doesnt work so get the fish back to the store where its more safe for them before you lose them.

Doubling your bioload is hell on your filter and fish. Especially with the pleco almost tripling it by themselves.

Well, I got a couple live plants and they snuck their way in that way. At their peak there were about 20 of them in the tank.... and I ended up liking the loach anyway. The other catfish died, but the rest look healthy so I suppose I'll just keep watching the water and wait for the water to cycle again, then add more tetras. Is there any chance I could put in a larger variety of fish? Since I don't have catfish any more, I only have the four tetras (will be six or eight), the loach, frog, and pleco. So are there fish that aren't schooling that would look good with these?

The store doesn't take returns of fish unless it's within two days, no exceptions. I don't have any decent aquarium stores near me. But this is great information. I just wish I knew this before I went to the store. They did tell me what I was doing would be working - I took a lot of their advice.